Gastrointestinal endoscopy is a widely practiced diagnostic technology in gastroenterology. New fiberendoscopes allow the inspection of the interior lining of the entire upper and lower gastrointestinal tracts and biopsies taken via the endoscope sample this mucosal surface. However, it is not possible to reach deep enough beneath it to examine the full thickness of the wall or the underlying structures. Although new imaging technology is available (percutaneous ultrasound, nuclear magnetic resonance, etc.), these techniques cannot image the intestinal wall to determine a diagnosis in most patients because of insufficient resolution, gut wall movement and inability to make transverse slices through localized areas identified at endoscopy. Furthermore, these techniques are expensive, slow, and of limited availability. We plan to investigate and develop an endoscopic miniature ultrasound imaging system mounted onto the tip of an endoscopic catheter which can be passed via the biopsy channel of available endoscopes. Because this system will use high frequency (15-20 MHz), it will yield excellent resolution (0.3mm) of the gut wall and the adjacent structures. The high resolution will be obtained at the expense of low penetration but this is not a problem because the probe is placed directly on the tissue to be imaged. The device will be easy to use, orient and interpret, with high resolution images. The device will be relatively inexpensive and therefore has an excellent cost benefit ratio. The investigation will involve several steps of which the first is to build a laboratory prototype. After the design considerations are resolved and the probe is tested in excised human tissue and in animals, an endoscopic prototype will be built. This device will be tested in animals and on excised tissue. It will then undergo clinical testing in patients with disease involving the full thickness of the intestinal wall. This includes Crohn's disease and neoplasms of the stomach, esophagus and colon. It may also be useful in physiologic studies of the wall musculature. This device will provide a new dimension in the diagnosis of gastrointestinal disease.